Well, but, to be fair, she never really shows an interest in things like dressing up, especially not in the beginning. Like in the Parasprites episode, yeah, she volunteered to help Rarity out by being a model, but obviously had NO interest in dressing up whatsoever. Plus her personality is sort of "Do whatever seems the best, most straightforward way of doing things" - not exactly suited for skirts and preening if you just want to get the job done nice and quick. Add in the music that tends to be paired with her in the show - rock music heavy on awesome guitar riffs - and her doing things like busting up barns and kicking dragons in the face - and you get a tomboy essence. I'm not sure what your definition of a tomboy is, but I'd find it hard to believe that you can't see where everyone else is coming from, at least a little. But, in my personal opinion, I think her being the spunky tomboy is nice - Being tomboyish myself, I may be biased, but tomboys tend to be active, straightforward, and a lot more likable for their energy and spunk. It gives them so much personality and vivacity to work with, and a tomboy can always be just as pretty as a fashionista, even without putting a lot of effort into her looks. Plus, it's a fun juxtaposition when you DO see her in a dress, because she acts as she always does, which can sometimes seem unsuited to the elegant dresses Rarity makes for her. xD